1. Field of the Invention
The present invention relates to a discharge-lamp lighting apparatus and a projector including the discharge-lamp lighting apparatus, and more particularly, to an operation of controlling a drive current at the start of the lamp.
2. Description of the Related Art
A discharge-lamp lighting apparatus is suggested in, for example, Japanese Patent No. 2942113 (claim 1). In this discharge-lamp lighting apparatus, while a lamp voltage is low in an initial stage of lighting, constant current control is performed in which a current supplied to the discharge lamp is controlled by a switching operation. Then, after the lamp voltage is stabilized, constant power control is performed in which electric power supplied to the discharge lamp is controlled by a switching operation. In this apparatus, a ratio of “on” period to “off” period of switching elements is controlled by changing a switching frequency of the switching elements. In addition, in an abnormal state, the switching frequency is set to a predetermined lower limit and the “on” period of the switching elements is reduced.
In the above-described known discharge-lamp lighting apparatus, after a high-voltage discharge lamp (hereinafter also called a lamp) is started, a constant drive current is supplied until a lamp voltage is increased and the lamp power reaches a rated power. Then, after the lamp power reaches the rated power, constant-power control is performed such that the lamp power is maintained constant. The lamp voltage depends on a pressure in an arc tube, and the pressure in the arc tube is increased as the temperature is increased due to light emission of the lamp and as the number of molecules is increased due to evaporation of mercury caused by the temperature increase. If the lamp has a secondary mirror, the temperature is further increased since the emitted light is returned by the secondary mirror, and therefore the pressure in the arc tube is rapidly increased. In this case, since a constant drive current is supplied, when the lamp voltage is rapidly increased, the lamp power is also increased rapidly. The rapid increase in the lamp power causes a rapid increase in a collision load placed on electrode tips by electrons in the arc tube, which leads to melting of the electrode tips. When the electrode tips melt, discharge arc is increased and the illumination is reduced.